Re: [RFC][Patch v8 6/7] KVM: Enables the kernel to isolate and report free pages

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Feb 5, 2019 at 3:21 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Feb 05, 2019 at 04:54:03PM -0500, Nitesh Narayan Lal wrote:
> >
> > On 2/5/19 3:45 PM, Michael S. Tsirkin wrote:
> > > On Mon, Feb 04, 2019 at 03:18:53PM -0500, Nitesh Narayan Lal wrote:
> > >> This patch enables the kernel to scan the per cpu array and
> > >> compress it by removing the repetitive/re-allocated pages.
> > >> Once the per cpu array is completely filled with pages in the
> > >> buddy it wakes up the kernel per cpu thread which re-scans the
> > >> entire per cpu array by acquiring a zone lock corresponding to
> > >> the page which is being scanned. If the page is still free and
> > >> present in the buddy it tries to isolate the page and adds it
> > >> to another per cpu array.
> > >>
> > >> Once this scanning process is complete and if there are any
> > >> isolated pages added to the new per cpu array kernel thread
> > >> invokes hyperlist_ready().
> > >>
> > >> In hyperlist_ready() a hypercall is made to report these pages to
> > >> the host using the virtio-balloon framework. In order to do so
> > >> another virtqueue 'hinting_vq' is added to the balloon framework.
> > >> As the host frees all the reported pages, the kernel thread returns
> > >> them back to the buddy.
> > >>
> > >> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
> > >
> > > This looks kind of like what early iterations of Wei's patches did.
> > >
> > > But this has lots of issues, for example you might end up with
> > > a hypercall per a 4K page.
> > > So in the end, he switched over to just reporting only
> > > MAX_ORDER - 1 pages.
> > You mean that I should only capture/attempt to isolate pages with order
> > MAX_ORDER - 1?
> > >
> > > Would that be a good idea for you too?
> > Will it help if we have a threshold value based on the amount of memory
> > captured instead of the number of entries/pages in the array?
>
> This is what Wei's patches do at least.

So in the solution I had posted I was looking more at
HUGETLB_PAGE_ORDER and above as the size of pages to provide the hints
on [1]. The advantage to doing that is that you can also avoid
fragmenting huge pages which in turn can cause what looks like a
memory leak as the memory subsystem attempts to reassemble huge
pages[2]. In my mind a 2MB page makes good sense in terms of the size
of things to be performing hints on as anything smaller than that is
going to just end up being a bunch of extra work and end up causing a
bunch of fragmentation.

The only issue with limiting things on an arbitrary boundary like that
is that you have to hook into the buddy allocator to catch the cases
where a page has been merged up into that range.

[1] https://lkml.org/lkml/2019/2/4/903
[2] https://blog.digitalocean.com/transparent-huge-pages-and-alternative-memory-allocators/